c# - ObservableCollection<T> on xaml -

i'm working on customconverter on wpf. generic convertion.

reading on blog, found way simplify xaml.

so converter looks this:

public customconverter : markupextension, imultivalueconverter {      public parameterscollection parameters { get; set; }      public object convert(object[] values, type targettype, object parameter, system.globalization.cultureinfo culture) {         if(parameters == null)             return binding.donothing;         //convertion logic         //...     }     public override object providevalue(iserviceprovider serviceprovider) {         return new customconverter();     } } public class parameterscollection : observablecollection<object> { } 

and in xaml file, have following:

<usercontrol xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"                   xmlns:primitives="clr-namespace:system;assembly=mscorlib"                  xmlns:converter="clr-namespace:ns.converters">     <label>         <label.visibility>             <multibinding>                 <converter:customconverter>                     <converter:customconverter.parameters>                         <converter:parameterscollection>                             <primitives:string>param1</primitives:string>                             ...                         </converter:parameterscollection>                     </converter:customconverter.parameters>                 </converter:customconverter>             </multibinding>             <!--bindings start here -->         </label.visibility>     </label> </usercontrol> 

so when debbuging code, parameters property empty (null), xaml not populating collection. question is, how popultate parameters using xaml, no c# code.

if put breakpoint on first line of convert method, you'll find parameters null. that's because never initialize it. solution initialize property in constructor (or right in declaration, if you're in c#6).

parameters doesn't need observablecollection<t>, because nobody ever bind it, , doesn't need non-generic subclass either. lastly, providevalue() should provide actual instance has parameters given xaml. if return new instance empty parameters collection, won't have parameters. providevalue() called after you're created , initialized. purpose isn't provide new instance of own class, markup extensions staticresource or dynamicresource take parameters , use create or retrieve instance of other themselves.

public class customconverter : markupextension, imultivalueconverter {     public customconverter()     {         parameters = new list<object>();     }     public list<object> parameters { get; set; }      public object convert(object[] values, type targettype, object parameter, system.globalization.cultureinfo culture)     {         if (parameters == null)             return binding.donothing;          //  returns something, testing         //  replace own convertion logic.          return string.join(";", parameters);     }      public object[] convertback(object value, type[] targettypes, object parameter, cultureinfo culture)     {         return null;     }      public override object providevalue(iserviceprovider serviceprovider)     {         return this;     } }